st: graph export does not seem to save very large images?

I'm having trouble saving very large images of multiple combined
graphs in Stata. If I go beyond a width of about 3500, I get the
following error:
. graph export histograms.png,width(5000) replace
could not create PNG file: histograms.png
r(693);
[r(693) looks like an IO error]
Is this a memory problem (I might have too little memory in this
computer)? The size of the images I'm able to save seems to vary
quite a bit depending upon what else is running. Does anyone know a
solution to this problem?
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/